It may be recalled that an osteosynthesis implant must serve to hold in place two (or more) parts of the same bone fractured or cut by a surgical operation (osteotomy), for the time necessary for the consolidation of this bone (typically 3 months).
It may be recalled that an arthrodesis is the blocking of a joint by surgery to fuse two bones into a single one, using an osteosynthesis device.
It may be recalled that the purpose of any osteosynthesis and particularly in the case of an arthrodesis is to obtain very good primary and also secondary stability, in order to obtain the consolidation in the best possible conditions, that is, in a position selected by the surgeon, by minimizing the problems of postoperative pain and edemas, by shortening the consolidation time as much as possible.
To obtain this result, the stability of the osteosynthesis associated with the implant is critical. Furthermore, the implant must also provide and maintain a slight compression on the portions of bone to be fused together, thereby facilitating this consolidation.
Various technical solutions have been proposed to carry out an arthrodesis, in particular at the tips (foot, hand, wrist, ankle, etc.).
Mention can be made, for example of basic staples which do not provide a proper fixation during consolidation, and shape memory staples which serve to place the two bone portions to be consolidated under compression, thereby corresponding to the intended purpose.
However, to obtain satisfactory stability, it is necessary to place two, or even three staples, in different planes. This significantly increases their size, thereby limiting the applications, in particular on small bones (for example in the fingers or toes).
It is also common to use extramedullary or extra osseous plates and screws, which also require a relatively large space and cannot be used on the terminal phalanges of the fingers (distal interphalangeal arthrodesis for example). Moreover, the medium term stability of these systems is not always guaranteed (loosening of the mounting).
Certain types of screw can be used in intramedullary cases, but in this case, the approach path requires a pulpar approach, which may generate serious complications (sepsis, etc.) and discomfort for the patient.
Use can also be made of pins which have a smaller size. However, the stability obtained is not satisfactory (problems of migration) and it is generally necessary to remove them after consolidation. With such devices, moreover, the patient cannot immerse the finger or toe treated, because the pin generally projects outside the skin.
Intramedullary osteosynthesis systems are available for long bones (tibia, femur, humerus, etc.). For example, lockable centromedullary nails are known. Apart from the fact that the locking technique is difficult, it cannot be miniaturized for extremity surgery (hand and foot).
Shape memory intramedullary devices are also available for solving part of the problems with respect to the arthrodesis or the osteosynthesis of the small fragments: for example, the solutions described in French patent 2 846 545 or French patent 2 884 406 (US 2008/0177262).
French Patent 2 846 545 describes an H-shaped device which opens in the body into an X, thanks to the use of a shape memory set around 37° C, each leg being implanted in a calibrated hole.
In practice, such a system does not allow proper introduction into the bone. This is because the preparation of 2 parallel holes in a phalange is extremely difficult due to the limited size and, above all, the parallel legs tend to open naturally during introduction and thereby exert an effect of distraction of the two fragments rather than compression.
Furthermore, the use of shape memory is very limiting due to the demands it makes on surgeons, in particular of temperature management: the implant must be fitted into the bone when cold before it warms and opens. This requires placing the implant in a support, storing it cold, and using all possible speed for implantation.
Finally, since the legs are straight, their shape memory tends to create a local support at their tips, which does not ensure satisfactory behavior and can damage the bone.
US 2008/0177262 teaches a system for easier introduction whether by the shape (eye) or by a support or a clamp that keeps the legs of the implant closed during introduction.
Nevertheless, these systems do not operate very dependably, because they do not define the optimal criteria allowing proper introduction into the bone and good anchorage: the anchoring zones always tend to open too early, thereby blocking introduction.